The field of the invention relates generally to radiographic instruments and more particularly to a scanning bone densitometer in which measurements of a patient are acquired in a series of overlapping scans.
Scanning radiographic equipment differs from conventional radiography in that it employs a narrowly collimated beam of radiation, typically x-rays, formed into, for example, a fan beam, rather than a broad area cone beam. The compact beam size allows the replacement of an image forming sheet of radiographic film, used with conventional radiographic equipment, with a small area array of electronic detector elements. Further the scanning allows the collection of data over a much broader area than would be practical with a single x-ray cone beam.
The electronic detector elements receiving the transmitted radiation produce electrical signals which may be converted to digital values by an analog to digital converter for the later development of an image or for other processing by computer equipment. The ability to quantify the measurement of the transmitted radiation, implicit in the digitization by the analog to digital converter, allows not only the formation of a radiographic "attenuation" image but also the mathematical analysis of the composition of the attenuating material by dual energy techniques. See generally, "Generalized Image Combinations in Dual KVP Digital Radiography" by Lehmann et al., Med. Phys. 8(5) Sept/Oct. 1981. Such dual energy techniques quantitatively compare the attenuation of radiation at two energies to distinguish, for example, between bone and soft tissue. This makes possible the measurement of bone mass, such measurement being important in the treatment of osteoporosis and other bone diseases.
The compact beam of radiation used in scanning radiographic systems allows the use of limited area detectors permitting high resolution with relatively lower cost. Further, the images formed by a compact beam are potentially more accurate than those produced by a typical broad beam radiographic system. The accuracy arises from the limited divergence of the rays of the beam as compared to a broad area cone beam. This narrow collimation of the fan beam reduces "parallax" in the projected image, particularly of anatomical planar surfaces that are nearly parallel with the central ray of the beam--such as the superior and inferior borders of the vertebra in the spine.
The compact beam of radiation, however, also requires increased scanning motion if large areas are to be measured. In a fan beam system, typically the fan beam will be scanned in a raster or "zig-zag" pattern over the area to be measured, each line of the scan forming a scan image separated by somewhat less than the width of the fan beam to ensure complete illumination of the entire volume of the imaged object. The direction of scanning is generally perpendicular to the direction of the radiation and the plane of the fan beam.
U.S. Pat. No. 5,305,368 entitled Method and Apparatus for Piece-Wise Radiographic Scanning issuing Apr. 19, 1994 assigned to the assignee of the present invention and hereby incorporated by reference, describes the creation of a single image, or quantitative data set, from a plurality of scan images acquired across a patient, either longitudinally aligned with the superior-inferior axis of the patient or transversely from the patient's left to right.
Transverse scanning provides the advantage that the time between the acquisition of adjacent scan images is minimized (because the transverse direction across the patient is shorter than the longitudinal direction across the patient). This reduces the severity of patient motion artifacts between scan images allowing them to be more accurately merged. A drawback to transverse scanning, however, is that the diverging beams of radiation used in the scanning creates triangular regions of overlap between adjacent scans.
As disclosed in the '368 patent, this overlap between adjacent scans distorts or blurs the imaged produced by combining the scan images near the edges of the scan images. The blurring is caused by a dependency of the projected image on the height of the imaged structure (the "height dependency problem") that displaces the relative position of the structure in the two scan image as a result of the different angles of illumination of the structure by the adjacent beams of radiation.
The '368 patent discloses weighting the edge portions of adjacent scan images to prevent a disproportionate contribution by redundant data of these overlapping edge portions to the combined image. The weighting assigns to the corresponding portions of two overlapping regions, weighting factors that sum to one. Each pixel of the overlapping region, may for example, be multiplied by a weighting factor of 0.5.
It has been suggested by some that a weighting system like that of the '368 patent, may solve the height dependency problem. However, as noted in the '368 patent, a weighting and combining alone necessarily blurs the image and is less preferred for diagnosis and measurement than some mis-registration in the combined image.
Accordingly, the '368 patent teaches the use of a longitudinal scanning in which the fan beams of adjacent scans are angled to eliminate the height dependency problem. Such a correction may be performed for longitudinal scans of the patient where a single wide angle fan beam may be used but is impractical for a transverse scanning of the patient where the creation of a single fan beam would entail too great an angular dispersion between individual rays of the beam.
Ideally, a scanning densitometry system might employ a combination of transverse scanning for the reduction of patient motion artifacts while reducing height dependency problems.